Desalination & Water Purification

Most ocean desalination facilities are located along the coast of the energy-rich Middle East. These technologies use heat evaporative systems, involve brine heaters, flash chambers and high temperature conversion processes in the conversion of seawater to fresh water. This technology has demanded the use of a wide range of materials involving the copper-base (cupronickels), iron-base (stainless steels) and titanium. Both the Multi-Stage Flash (MSF) and the Multi-Effect Distallation (MED) processes are very capital intensive, with large footprints.

The membrane process, or reverse osmosis (RO), is the low-temperature, high-pressure process in achieving the same ends. At the same time, this is a separation process used in the optimization and purification of potable and drinking waters. This process can be modularized or built as a full-scale plant for conversion.

The membrane systems are continually being performance optimized, resulting in lower pressure requirements for conversion. Today, seawater to fresh water conversion pressures are typically below 600 psi. These forces are required to provide the high, hydraulic driving force for salt separation at the membrane interface.

Seawater and brackish water RO-conversion to fresh waters involves an extensive use of iron-based, stainless steel pipelines and ancillary equipment, depending upon the concentration of chlorides that are being handled. Typically, Type 316 stainless steel pipe systems are used for the processed-side fresh waters, while the higher corrosion resistant materials, such as the duplex stainless steels and super-austenitic materials, handle the higher chloride-containing and brine water streams, for compositions (see Table 2). Some of these corrosion resistant materials also have to be used for handling the cleaning and backwash waters of the membranes. These streams are acidic in nature, consisting of buffered organic acids and small quantities of hydrochloric acid.

Common to both the high temperature and low temperature conversion technologies, common materials for pump and valve bodies involve cast austenitic and duplex grades of stainless steels, while nickel-aluminum bronze has been selected for the gates and some internals. Cast duplex stainless steel (typically CD4MCu) has been used for pump internals for their corrosion-erosion and wear resistance in handling slurries and brines.

Membrane cell technology is also being used for the purification and upgrading of potable and drinking waters. Management of these water streams involves Type 316 stainless steel for their piping systems. These involve micro-filtration, ultra-filtration and nano-filtration membrane processes. These can be used separately or in combination with RO membrane cells for water conversion and purification.

In the case of microfiltration, the technology is used to remove microbes and coliform from water streams, especially Giardia and Cryptosporidia in the processing of potable and drinking water streams. This is often used in conjunction with the RO process.

Ultrafiltration is a membrane process for the removal of natural organic matter, the efficiency of which can be enhanced in conjunction with powdered activated carbon as a pre-treatment filter. These membrane systems address the reduction and removal of natural organic matter, including trihalomethane (THM),, which may be present in surface and lake waters.

Nanofiltration and the RO process are capable of removing dissolved molecules and ions, along with control for the total organic carbon (TOC) levels to meet the Disinfectants/Disinfection Rules, which controls the maximum contamination levels permitted in finished waters.

In all of these cases, acid cleaning and backwashing of the membrane surface is a necessary and regular practice in order to maintain performance efficiencies, which require the use of higher corrosion-resistant grades of iron-based stainless steel materials for piping and handling systems.

RO treatment of waste water streams is also assuming a major significance in the supply of treated process waters for industrial, commercial and recreational reuse. This water reuse can complement the availability of treated waters, especially in dual supply lines, where a smaller pipeline can be used to supply potable and drinking waters from the water treatment plant and a larger line handling water reuse for all other purposes.